1. Field of the Invention
The present invention generally relates to body-implantable devices. More particularly, the present invention relates to a percutaneously insertable and expandable inter-vertebral disc prosthesis. Specifically, the present invention comprises a novel nuclear prosthesis, a specially designed delivery apparatus, and a loading apparatus for loading the nuclear prosthesis within the delivery apparatus.
2. Description of the Related Art
The role of the inter-vertebral disc in spine biomechanics has been the subject of extensive research and is generally well understood. A typical native spinal unit is shown for exemplary purposes in FIG. 22A. The functional spinal unit, or spinal motion segment 500 consists of two adjacent vertebrae 502 and 504, the inter-vertebral disc 506 and the adjacent ligaments (not shown). The components of the disc are the nucleus pulposus 506a, the annulus fibrosis 506b, and the vertebral end-plates 506c. These components act in synchrony and their integrity is crucial for optimal disc function. During axial loading of the normal native disc 506, the pressure of the nucleus pulposus 506a rises, transmitting vertical force on the end plates 506c and outward radial stress on the annulus fibrosis 506b, as shown by the direction arrows in FIG. 22A. The vertical stress is transformed to tensile forces in the fibers of the annulus fibrosis 506b. Because the gelatinous nucleus pulposus 506a is deformable but noncompressible, it flattens radially, and the annulus fibrosis 506b bulges and stretches uniformly. Flexion of the spine involves the compression of the anterior annulus fibrosis 506b, as well as the nucleus pulposus 506a. The nucleus pulposus 506a deforms and migrates, posteriorly stretching the annular fibers and expanding radially. Thus, the nucleus pulposus 506a and annulus fibrosis 506b function synergistically as a cushion by reorienting vertical forces radially in a centrifugal direction.
The native vertebral end plate 506c prevents the nucleus pulposus from bulging into the adjacent vertebral body by absorbing considerable hydrostatic pressure that develops from mechanical loading of the spine. The end plate 506c is a thin layer of hyaline fibrocartilage with subchondral bone plate, typically around 1 millimeter thick. The outer 30% of the end plate 506c consists of dense cortical bone and is the strongest area of the end plate 506c. The end plate 506c is thinnest and weakest in the central region adjacent to the nucleus pulposus.
With aging and repetitive trauma, the components of the inter-vertebral disc 506 undergo biochemical and biomechanical changes and can no longer function effectively, resulting in a weakened inter-vertebral disc 506. As the disc 506 desiccates and becomes less deformable, the physical and functional distinction between the nucleus pulposus 506a and the annulus fibrosis 506b becomes less apparent. Disc desiccation is associated with loss of disc space height and pressure. The annulus fibrosis 506b loses its elasticity. The apparent strength of the vertebral end-plates 506e decreases and vertebral bone density and strength are diminished. This leads the end-plates 506c to bow into the vertebral body, imparting a biconcave configuration to the vertebral body. Uneven stresses are created on the end plates 506c, annulus fibrosis 506b, ligaments (not shown), and facet joints (not shown), leading to back pain. At this point, the annulus fibrosis 506b assumes an inordinate burden of tensile loading and stress, and this further accelerates the process of degeneration of the annulus fibrosis 506b. Fissuring of the annulus fibrosis 506b further diminishes its elastic recoil, preventing the annulus fibrosis 506b from functioning as a shock absorber. Leakage of the nuclear material can cause irritation of the nerve roots by both mechanical and biochemical means. Eventually, degenerative instability is created, leading to both spinal canal and neuroforaminal stenosis.
Historically, spine surgery consisted of simple decompressive procedures. The advent of spinal fusion and the proliferation of surgical instrumentation and implants has led to an exponential utilization of expensive new technologies. As an alternative to open surgical discectomy and fusion, Minimally Invasive Spinal Surgery (MISS) has been advocated. Thus far, the primary rationale for favoring the MISS approach has been to lessen postoperative pain, limit the collateral damage to the surrounding tissues, and hasten the recovery process rather than affect long term outcomes. Despite the lack of clear superiority and outcome data, these technologies have continued to flourish.
However, many spinal surgeons remain skeptical about the positive claims regarding MISS, citing certain drawbacks, including increase in operating room time, requirement for expensive proprietary instruments, increased cost, and the technically demanding nature of the procedure. Despite the advantage of a minimal incision approach, MISS requires an adequate decompression and/or fusion procedure in order to have results comparable to traditional open surgical approaches.
Ideally, a nuclectomy and implant insertion would be performed through a percutaneous posterolateral approach. Advantages of the percutaneous posterolateral approach over conventional open surgery and MISS include obviating the need for surgically exposing, excising, removing, or injuring interposed tissues; preservation of epidural fat; avoiding epidural scarring, blood loss, and nerve root trauma. Other advantages include minimizing “access surgery” and hospitalization costs, and accelerating recovery. A percutaneous procedure may be expeditiously used on an outpatient basis in selected patients. On the other hand, percutaneous insertion imposes a number of stringent requirements on the nuclear prosthesis and its method of delivery.
Several devices have been used to fill the inter-vertebral space void following discectomy in order to prevent disc space collapse. These devices generally fall into two categories: fusion prostheses and motion prostheses. Fusion prostheses intended for MISS insertion offer few if any advantages over those for open surgical technique. While these types of implants eliminate pathological motion, they also prevent normal biomechanical motion at the treated segment. Greater degrees of stress are transmitted above and below the treated segment, often leading to accelerated degeneration of adjacent discs, facet joints, and ligaments (adjacent level degeneration).
Motion prostheses generally aim at restoring disc height, shock absorption, and range of motion, thus alleviating pain. Artificial motion prostheses may be divided into two general types: the total disc prosthesis and the nucleus prosthesis. The total disc prosthesis is designed for surgical insertion, replacing the entire disc, while the nucleus prosthesis is designed for replacing only the nucleus pulposus, and generally may be inserted by open surgical or MISS methods.
Prior designs of motion nucleus prostheses include enclosures that are filled with a diverse variety of materials to restore and preserve disc space height while permitting natural motion. However, there are several shortcomings of prior nucleus motion prostheses designs. Some of the prior nucleus motion prostheses require surgical approaches for insertion that involve removal of a significant amount of structural spinal elements including the annulus fibrosis. Removal of these structural spinal elements causes destabilization of the spinal segment. Prior nuclear motion prosthesis designs also fail to provide the outer margin of the nuclear prosthesis with surface and structural properties that encourage native tissue ingrowth. Instead, such prostheses are made from generally non-porous materials that impede full incorporation of the nuclear prosthesis into the surrounding annular margin.
Some prior designs have annular bands along the outer periphery of the nucleus motion prostheses. However, prior annular bands are non-compliant. This is disadvantageous because it reduces the radial outer expansion required for load dampening. Thus, the load is transferred to the end plates of the vertebrae, which can withstand only limited deformation. The result is that the end plates eventually fail, resulting in loss of intradiscal pressure, accelerated degeneration, and subsidence of the nuclear prosthesis. Other prostheses do not have an annular band. These prostheses tend to exert untoward pressure on an already weakened annulus fibrosis. Particularly, such a prosthesis tends to protrude into a pre-existing annular tear.
Other designs fail to incorporate or use a central gas cushion with a valve system or assembly that does not leak. Still others concentrate the harder load bearing component of the nuclear prosthesis in the central aspect of the disc, predisposing the nuclear prosthesis to subsidence. Another problem with prior nuclear motion prostheses is the imprecise sizing and tailoring of the nuclear prosthesis. Over sizing places unnecessary stress on the already damages and degenerated annulus fibrosis, while under sizing of the nuclear prosthesis may result in inadequate contact with the inner wall of the annulus fibrosis, and possibly non-integration and migration of the nuclear prosthesis. Other designs of nucleus motion prostheses suffer draw backs such as bulkiness, inelasticity, inability to fold and pack the nuclear prosthesis into a delivery cannula or apparatus for percutaneous implantation into a patient. In fact, percutaneous delivery of a motion nucleus prosthesis heretofore, has been unavailable.
Applicants here propose to overcome the disadvantages of the prior designs of nucleus motion prostheses by providing a multi-compartment nuclear prosthesis having a semi-compliant annular reinforcement band disposed adjacent or contiguously around the periphery of a rubber filled annular enclosure. The annular enclosure nests a central, gas cushioned nuclear enclosure and an integrated sealing valve assembly. The nuclear prosthesis of the present invention is foldable to fit within a delivery apparatus, and is intended for percutaneous insertion into a nuclear space void following percutaneous total nuclectomy. Once percutaneously inserted, the nuclear prosthesis is expandable by an inflation-assisting device to provide cushioning and stability to a spinal segment weakened by degeneration.